1. Field of the Invention
The present invention is related to optical transducers for reading dark coding marks on paper media. More particularly the present invention is related to a way of sensing partially opaque pencil marks on paper cards which does not depend on reflected light as employed in the prior art.
2. Description of the Prior Art
Mark sense data cards are well known and have been in use so long that generally accepted specifications exist for the size and spacing of the marks as well as the hardness of the pencils to be employed for making the marks. Because the data card or paper used has a rough finish, pencil marks do not completely cover or opaque the paper in the marked zone. Also, the pencil mark does not absorb one hundred percent of the light. Such marks are partially opaque and some light will pass through the mark. If the paper without a mark was taken as a perfect reflector and the pencil mark reduced the reflected light by fifty percent, the light to dark ratio would be two to one and could be readily sensed.
Mark sensors of the prior art reflective type require that the card be read in an exact predetermined focal plane because the light source is concentrated on a spot embracing the mark to be sensed and the detector must detect the light reflected from the same predetermined mark, otherwise stray light and diffused light would reduce the accuracy of the light to dark ratio being sensed.
The variation of a card from an optimum focal plane is referred to as card flutter. Manufacturers of optical reflective transducers publish curves which show preferred card to sensor distances and tolerances.
Due to flutter, reflectivity of the card or paper in the mark zone, differences in the absorption of the marks made, as well as aging of the light source, the optimum light to dark ratio of the reflective type sensors have not been uniform or consistently accurate.
Marks may be sensed on cards or objects by transmitting light completely through the paper and sensing the change of light transmitted through the paper. This method of sensing is only applicable where a special translucent calibrated paper is employed.